It is a brief walk to the Boardwalk from the International Gateway, maybe 10 minutes at a very casual stroll....but you need to get yourself there (Near the UK in World Showcase) so it will depend a lot upon what part of the park you will be in before heading to your reservation.....so if you are at Mission Space for example, you'll have quite a bit of walking across the park before setting off to the Boardwalk. Take a look at the maps, and see what your morning plans are if you have any fastpass+ reservations to judge how much time you need.